In a wireless communications system wherein the transmitted signal is spread via a code sequence, such as in a direct-sequence code-division multiple access (DS-CDMA or simply, CDMA) communications system; there is a requirement that the spreading waveform used at the transmitter and the receiver be synchronized. If the spreading waveforms are out of synchronization (even by as small an amount as a single chip (a chip is the smallest transmission unit)), then the receiver's data demodulator may receive insufficient signal energy to reliably detect (and decode) the transmitted data. One cause for lack of synchrony in transmitter and receiver spreading waveforms is code-phase uncertainty.
There are typically two components to code-phase uncertainty. A first component to code-phase uncertainty involves the determination of an initial code phase, commonly referred to as code acquisition or initial acquisition. The code acquisition is normally achieved by a component of a wireless communications device known as the searcher during the initial stages of communications. A second component to code-phase uncertainty is commonly referred to as a code tracking loop. The task of the code tracking loop is to maintain code synchronization after the code acquisition.
A delay lock loop (DLL) is one commonly used algorithm for the code tracking loop. The DLL makes use of a correlation operation between the received signal and two different code phases, an early and a late phase, of the receiver spreading waveform. More advanced DLLs can make use of on-time energy to normalize the early and the late signals. The DLL compares the early signal sample with the late signal sample and if the difference between the two is above a certain threshold, the DLL can then adjust the sampling point in an advance (early) or retard (late) direction.
The wireless communications channel can be modeled by its time and frequency varying impulse response. The communications channel's coherence time and coherence bandwidth can indicate how fast the channel is varying in time and frequency, with the coherence time being related to the Doppler spread (a function of the wireless device's speed) while the coherence bandwidth is related to multipath fading and is a function of the delay spread. For wideband communications systems, such as wideband CDMA (W-CDMA) the delay spread may be inherent due to the short pulse duration of the communications system.
A commonly used technique for processing multipath involves the use of a rake receiver (with multiple fingers), which can process the received signal with a certain multipath delay. A DLL can be implemented in each finger to make each multipath synchronized. With each finger assigned to a multipath, the received signal can be formed by combining the various multipaths, thus improving the quality of the received signal as a whole.
One disadvantage of the prior art is the occurrence of what is known as fat paths, or multiple paths that are close to one another, with a separation of one chip or less. With the multipaths being so close, the decision from the DLL (early, on-time, or late) can be interfered with because adjacent multipath signals may overlap one another. This may result in a loss in the synchrony between transmitter and receiver spreading waveforms.